This invention relates to a holder for compound semiconductor wafers, such as gallium arsenide (GaAs) and indium phosphide (InP), during molecular beam epitaxial (MBE) growth. A holder is required during MBE growth for three reasons. First, it supports and protects the very brittle wafer from breakage during handling within the MBE system. Second, it serves as a hot plate to enable heating to temperatures for MBE growth (500-700 degrees Celsius). Third, it prevents loss of the more volatile element of the compound (eg. arsenic in GaAs) from the backside of the wafer during heating.
Prior art holders for GaAs wafers can be divided into two types. The first type of holder includes a plate made of molybdenum onto which the wafer is soldered with indium. The molybdenum plate serves as mechanical protection for the GaAs wafer. The radiation heater inside the MBE system heats the molybdenum plate and the indium serves as a medium for heat transfer to the GaAs wafer. Since indium is molten in the temperature range of interest, it will relieve any stress caused by the difference in thermal expansion between the GaAs wafer and the molybdenum plate. The surface tension of the molten indium also holds the GaAs wafer in place. While the indium holder has been in use for about ten years, there are several disadvantages to its use. The mounting and demounting of wafers using indium solder is impractical, time consuming and often causes the wafers to break. It is difficult to get uniform indium coverage under a larger wafer, and nonuniform coverage will result in nonuniform heating of the GaAs wafer and also causes the GaAs wafer to bow severely during growth. Finally, the indium has to be removed from the backside of the wafer after MBE growth prior to any further processing. This is often difficult since indium may be incorporated deep into the backside of the wafer during MBE growth.
A more recent technique that avoids the use and problems of indium uses a thin, evaporated refractory metal film on the backside of the wafer. The metal film now serves as the hot plate and transfers the heat to the GaAs wafer. The metal film also acts as a cap to prevent arsenic loss from the backside of the wafer. A molybdenum ring-shaped holder is used for mechanical protection. While this technique offers advantages over the indium technique, it, too, has disadvantages. Deposition of the refractory metal adds an additional process step during preparation of the wafer prior to MBE growth. Like the indium, this refractory metal film must be removed after MBE growth. Furthermore, the metal film also causes the wafer to bow during growth.
Therefore, it is an object of the present invention to provide a GaAs wafer holder that does not require processing of the wafer before or after MBE growth.
It is another object of the present invention to provide a GaAs wafer holder that minimizes bowing of the wafer during MBE growth.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.